1. Field of the Invention
The present invention relates to a semiconductor device using an oxide semiconductor and a method for manufacturing the semiconductor device.
2. Description of the Related Art
In recent years, a technique in which a thin film transistor (also referred to as a TFT) is formed using an oxide semiconductor and is applied to an electronic device and the like has attracted attention. For example, a technique in which a switching element and the like of an image display device are formed using zinc oxide, an In—Ga—Zn—O-based oxide semiconductor, or the like as an oxide semiconductor layer is disclosed in Reference 1 and Reference 2.
In addition, as a structure of a transistor using an oxide semiconductor layer, various structures are proposed. For example, a bottom-gate bottom-contact structure, in which an oxide semiconductor layer is formed over a source electrode layer and a drain electrode layer which are provided over a gate insulating layer, is disclosed in Reference 2 and Reference 3.
[REFERENCES]
Reference 1: Japanese Published Patent Application No. 2007-123861
Reference 2: Japanese Published Patent Application No. 2007-96055
Reference 3: Japanese Published Patent Application No. 2007-305658
In the case where a source electrode layer and a drain electrode layer are formed over a gate insulating layer and then an oxide semiconductor layer is formed over the gate insulating layer, the source electrode layer, and the drain electrode layer, the oxide semiconductor layer is formed after the source electrode layer and the drain electrode layer are formed. In this case, before the oxide semiconductor layer is formed, element characteristics might deteriorate due to impurities attached to the surfaces of the gate insulating layer, the source electrode layer, and the drain electrode layer. In addition, if a film including impurities is formed on the surfaces of the gate insulating layer, the source electrode layer, and the drain electrode layer before the oxide semiconductor layer is formed, the element characteristics might deteriorate.
For example, in the case where a source electrode layer and a drain electrode layer are formed by a photolithography method, a resist or a resist stripping solution is in contact with a gate insulating layer, the source electrode layer, and the drain electrode layer; therefore, impurities might be attached to surfaces or a film including impurities might be formed on the surfaces in some cases. In addition, even when a source electrode layer and a drain electrode layer are selectively formed over a gate insulating layer by a droplet discharge method such as ink-jet printing, an additive such as a solvent or a dispersant included in ink might be in contact with the surfaces of the gate insulating layer, the source electrode layer, and the drain electrode layer and a film including impurities might be formed in some cases.
Further, in the case where metal is used as a source electrode layer and a drain electrode layer, after the source electrode layer and the drain electrode layer are formed and before an oxide semiconductor layer is formed, the surfaces of the source electrode layer and the drain electrode layer might be oxidized, so that contact resistances between the source electrode layer and the oxide semiconductor layer and between the drain electrode layer and the oxide semiconductor layer might be increased and the element characteristics might deteriorate.
In the case where an oxide semiconductor layer is formed over a source electrode layer and a drain electrode layer which are provided over a gate insulating layer, if the source electrode layer and the drain electrode layer each have a large thickness and unevenness, disconnection of the oxide semiconductor layer formed over the gate insulating layer, the source electrode layer, and the drain electrode layer might occur and the element characteristics might deteriorate in some cases.
In view of the foregoing problems, an object is to suppress deterioration of element characteristics even when an oxide semiconductor is formed after a gate insulating layer, a source electrode layer, and a drain electrode layer are formed.